1. Technical Field
The present invention relates to an optical fiber and a method for manufacturing silica glass. More particularly, the present invention relates to an optical fiber and a method for manufacturing silica glass that achieve low attenuation and are suitable for wavelength division multiplex (WDM) within the wavelength range of 1300 nm to 1625 nm.
2. Related Art
The widespread use of the Internet has been rapidly increasing the amount of information communicated. Therefore, it has been desired to improve the transmission capacity of optical fiber communication systems. Coarse wavelength division multiplexing (CWDM) is a technique for simultaneously transmitting a plurality of optical signals with different wavelengths within the wavelength range of 1300 nm to 1625 nm using the same fiber. This technique, in principle, achieves improved transmission capacity, which is equal to the result of multiplying the transmission capacity of single-wavelength transmission by the number of the wavelengths that enter at the same time. Here, a silica glass-based optical fiber is highly transparent at the wavelengths of 1300 nm to 1600 nm and generally has attenuation of 0.4 dB/km or less. The attenuation is dependent on the wavelength and is expressed by the following Expression 1, where α denotes the attenuation and λ denotes the wavelength.
                    α        =                              A                          λ              4                                +          B          +                      C            ⁡                          (              λ              )                                                          Expression        ⁢                                  ⁢        1            
In Expression 1, the first term on the right side denotes the Rayleigh scattering loss, the second term denotes the structural-imperfection-induced loss, and the third term denotes the absorption loss caused by metal impurities and OH groups.
A conventional silica glass-based optical fiber has a lot of OH groups mixed therein that have an absorption peak in the vicinity of the wavelength of 1383 nm. This makes it difficult to use the optical fiber for WDM transmission within the wavelength range of 1300 nm to 1625 nm. To address this issue, Patent Document 1 discloses a method of fabricating silica glass for an optical fiber having the smallest possible number of OH groups mixed therein and thereby achieving reduced absorption loss at 1383 nm. Furthermore, it is known that the attenuation of the silica glass-based optical fiber may increases in the vicinity of 1383 nm when hydrogen is diffused within the silica glass-based optical fiber (see Non-Patent Documents 1 and 2).
During the step of spinning an optical fiber base material into an optical fiber, the base material made of silica glass is exposed to high temperature and elongated with high tensile force. Here, it is believed that the base material is rapidly cooled down with its glass structure being broken to generate structural defects, which are generally represented by Expression 2 and referred to as non-bridging oxygen hole centers: NBOHCs).═Si—O.  Expression 2
Here, it is known that the concentration of the NBOHCs in the optical fiber is dependent on the tensile force and the cooling rate during the spinning step. It is also known that the concentration of NBOHCs increases as the tensile force or cooling rate increases during the spinning step.
Furthermore, it is known that hydrogen molecules, which are small, are easily diffused at room temperature within the glass structure of the silica glass of which the optical fiber is made. If hydrogen molecules are diffused within the silica glass, the hydrogen molecules react with NBOHCs to generate OH groups as shown by Expression 3. This results in absorption loss in the vicinity of 1383 nm.═Si—O.+½H2→═Si—OH  Expression 3
To prevent such degradation in absorption loss of the optical fiber made of silica glass, the optical fiber may be exposed to a deuterium atmosphere. According to this method, instead of hydrogen, deuterium, which is an isotope of hydrogen, is diffused within the optical fiber to react with the NBOHCs as expressed in Expression 4.═Si—O.+½D2→═Si—OD  Expression 4
If the NBOHC defects disappear in this way, hydrogen may later diffuse within the silica glass but does not cause the OH group-induced increase in absorption loss. This reaction easily proceeds at room temperature as disclosed in Patent Document 2. The generated OD groups do not have absorption loss in the wavelength range of 1300 nm to 1625 nm. Therefore, the attenuation in this wavelength range is hardly affected. Accordingly, the method using deuterium is effective in fabricating silica glass optical fibers with low attenuation.
In some occasions, however, the absorption loss may increase in the vicinity of the wavelength of 1400 nm after the deuterium treatment as shown in FIG. 1. It has been proved that this increase in absorption loss is unstable and is likely to decrease as the time elapses and ultimately substantially disappears as shown in FIG. 2. It, however, takes approximately two to three months until the increase in absorption loss disappears. Therefore, such an increase in absorption loss significantly hinders the optical fiber manufacturing. Here, FIG. 1 shows the relation between the wavelength and the absorption loss for an optical fiber that has been treated with deuterium and an untreated optical fiber. Curve 1 represents the attenuation spectrum of the optical fiber that has been treated with deuterium, and Curve 2 represents the attenuation spectrum of the untreated optical fiber. FIG. 2 shows the relation between the days that have elapsed and the absorption loss at the wavelength of 1400 nm.
Patent Document 3 introduces a hypothesis that the increase in absorption loss at the wavelength of 1400 nm may result from per-oxy linkages (POLs) in the silica glass. When silica glass base materials fabricated under the same conditions are spun into optical fibers, the amount of the increase in absorption loss may vary depending on the spinning conditions. It is, however, not clear how the spinning conditions are related to the amount of POLs generated. The increase in absorption loss at the wavelength of 1400 nm has thus not yet been clarified. Patent Document 3 discloses an optical fiber that achieves reduced increase in absorption loss at the wavelength of 1400 nm. This optical fiber is realized by a low-productivity method that involves a low drawing speed and requires manufacturing condition optimization based on electron spin resonance evaluation.
Patent Document 1: Japanese Patent 3970692
Patent Document 2: EP 1182176 B1
Patent Document 3: Japanese Patent Application Publication No. 2006-030655
Non-Patent Document 1: “New Hydrogen Aging Loss Mechanism in the 1400 nm Window,” K. H. Chang, D. Kalish and M. L. Pearsall; Proceedings OFC 99.
Non-Patent Document 2: “Formation of Hydroxyl Due to Reaction of Hydrogen with Silica in Optical Finer Preforms,” J. Stone, J. M. Wiesenfeld, D. Marcuse, C. A. Burrus and S. Yang; Apllied Physics Letters 47, No. 3, 328-330, 1 Aug. 1985.
In light of the above-described problems, an object of the present invention is to provide a method for manufacturing silica glass that can reduce the increase in absorption loss in the vicinity of the wavelength of 1400 nm that is caused by deuterium treatment and can efficiently fabricate a silica glass optical fiber having low attenuation in the wavelength range of 1300 nm to 1625 nm, and to provide such a silica glass optical fiber.